This invention relates to the field of signal processing. More particularly, the present invention relates to methods and apparatus for signal filtering.
One task that often is performed in signal processing is the removal of a portion of a signal, based on the specific characteristics of that portion of the signal. An analog or digital signal processing device or method that performs this function is often referred to in the art as a filter.
Another function that can be performed by a filter is signal separation. Often a filter is employed to remove a part of a signal and output one component of the signal and then that first output component is subtracted from the original signal to produce an additional separated component.
One application for signal filtering and signal separation is decoding a composite color television signal. As is known in the art, several common television encoding standards (such as NTSC), encode color television signals into two principal components, Y (luminance) and C (chrominance) and combine these two components into a composite video signal. For broadcast, this signal is then applied to an RF carrier frequency. The luminance component carries the primary gray-level information and is suitable for display on a black and white television. The chrominance component carries color difference information and is decoded and combined with the Y component to produce RGB control signals for RGB color television reproduction.
A comb filter is one type of advanced filter used to separate a composite signal into Y and C components, while maintaining bandwidth. A comb filter is ideal for composite signal with no signal variations in vertical direction as well as for signal of a transmitted still picture. It does not affect resolution in horizontal direction.
U.S. Pat. No. 5,155,582 , entitled DOT CRAWLING INTERFERENCE ELIMINATION DEVICE AND COLOR SUBCARRIER VERTICAL CORRELATION DETECTION DEVICE, issued Oct. 13, 1992 , discusses a number of problems and issues related to decoding composite television signals:
There are various problems occurring in a television receiver, such as dot crawling interference and cross color interference which occur due to multiplexing of the frequency of a color signal with the frequency of a luminance signal, line flicker which occurs due to interlace scanning, and deterioration of picture quality which occurs due to reduction in vertical resolution, etc. In order to solve these problems, frame memories and digital signal processing techniques have been used in recent years, thereby to improve the quality of the picture in a television receiver. These methods introduce processing techniques such as a three-dimensional Y/C separation which utilizes correlation of time directions of video signals and a sequential scanning converter.
The ""582 patent discusses a dot crawling interference elimination device with a means for detecting a vertical correlation from the sum of interline color subcarrier of an input video signal, and a notch filter for eliminating dot crawling interference from a luminance signal resulting from a Y/C separation of the input signal, in accordance with an output of said detecting means. The color subcarrier vertical correlation detection device includes means for detecting vertical correlation of a color burst signal of an input video signal and means for discriminating a non-standard state of the input video signal on the basis of an output of the detecting. The discussed circuit of the patent is directed to detecting a vertical correlation of a color subcarrier of a non-standard signal and applies a notch filter to a luminance signal resulting from a Y/C separation in accordance with a result of the detection, so that a satisfactory luminance signal having suppressed dot crawling interference can be obtained.
U.S. Pat. No. 5,594,508 discusses several preset filters that can be changed or switched based on a correlation analysis.
U.S. Pat. No. 4,814,863 discusses switching comb and notch filters using artifact analysis criteria.
Publication WO 93/10641 describes nonlinear signal processing wherein a color envelope signal is obtained to control subtraction of chroma signal for a composite signal.
Video Technology Explained, by Steve Mullen (1999), reviews a number of relevant background issues regarding digital sampling time base correction, luminance noise reduction, chroma noise reduction, field memory noise reduction, color bleed reduction, luminance delay adjustment and Y/C separation.
Guide to Comb Filters: Y/C Separation, by Greg Rogers (http://www.cybertheater.com/ Tech_Reports/Comb_Filter_Tut/guide_comb_filters.html reviews background concepts related to Y/C separation and comb filters.
While many different methods and systems for separating a composite signal have been proposed, less complex systems suffer from the artifacts and imperfections just described. Complex signal processing systems that can eliminate these artifacts, are generally too expensive to implement in many consumer video systems. What is needed is a relatively simple method or device that can separate a composite signal without unwanted artifacts.
The present invention in various embodiments encompasses a method or apparatus for improved signal filtering. According to the invention, an information signal is processed by a 2D (two-dimensional) filtering system designed so that in several consecutive steps a desired separated signal is obtained. In one embodiment, the invention is a method for using a low pass filter architecture to filter composite TV signal in a vertical direction to produce a low pass filter output that contains a low frequency component and an alias higher frequency component. A filter in a horizontal direction is then used to generate a desired luminance or chrominance signal.
The invention has particular applications in generating a luminance component signal from a composite color television signal and also may have other applications.
Among other benefits, the present invention reduces unwanted interference of a luminance and a chrominance television signals when vertical/diagonal lines occur. Consecutive filtering eliminates possibility of the vertical lines spectrum being demodulated as a color signal and thereby degrading displayed image quality.
It is known in the art that signal processing may be accomplished by analog circuits, by digital circuits, or by logic processes executed on computing devices, and specific embodiments of the present invention encompass each of the various ways known in the art for signal processing. Terms such as xe2x80x9ca filterxe2x80x9d xe2x80x9can adderxe2x80x9d xe2x80x9ca multiplierxe2x80x9d, etc., may be understood in the art as referring to either an analog circuit device or part thereof, a digital circuit device or part thereof, or a logical process or operation in a general purpose computer system or a special-purpose signal processing system. When used in reference with the present invention or in the claims, it is intended that these terms may incorporate any of these meanings unless the context requires otherwise.